diff --git a/parser.js b/parser.js
index f8c4d89..a144eb3 100644
--- a/parser.js
+++ b/parser.js
@@ -1,40 +1,57 @@
 const {dom, rule, ruleset} = require('fathom-web');
 const DEFAULT_RULESET = require('./lib/default-ruleset');
 
-const MetadataParser = {
-  metadataRules: Object.assign({}, DEFAULT_RULESET),
-  buildRuleset(name, rules) {
-    const reversedRules = Array.from(rules).reverse();
-    const builtRuleset = ruleset(...reversedRules.map(([query, handler], order) => rule(
-      dom(query),
-      node => [{
-        score: order,
-        flavor: name,
-        notes: handler(node),
-      }]
-    )));
+function buildRuleset(name, rules) {
+  const reversedRules = Array.from(rules).reverse();
+  const builtRuleset = ruleset(...reversedRules.map(([query, handler], order) => rule(
+    dom(query),
+    node => [{
+      score: order,
+      flavor: name,
+      notes: handler(node),
+    }]
+  )));
 
-    return doc => {
-      const kb = builtRuleset.score(doc);
-      const maxNode = kb.max(name);
-      if (maxNode) {
-        const value = maxNode.flavors.get(name);
-        if (value) {
-          return value.trim();
-        }
+  return doc => {
+    const kb = builtRuleset.score(doc);
+    const maxNode = kb.max(name);
+    if (maxNode) {
+      const value = maxNode.flavors.get(name);
+      if (value) {
+        return value.trim();
       }
-    };
-  },
-  getMetadata(doc) {
-    const metadata = {};
-
-    Object.keys(MetadataParser.metadataRules).forEach(metadataKey => {
-      const metadataRule = MetadataParser.buildRuleset(metadataKey, MetadataParser.metadataRules[metadataKey]);
-      metadata[metadataKey] = metadataRule(doc);
-    });
+    }
+  };
+}
 
+function getMetadata(doc, rules) {
+  return Object.keys(rules).reduce((metadata, key) => {
+    metadata[key] = buildRuleset(key, rules[key])(doc);
     return metadata;
+  }, {});
+}
+
+class MetadataParser {
+  constructor(customRules = {}, options = {}) {
+    this._rules = options.replace ? customRules : Object.assign({}, MetadataParser.metadataRules, customRules);
+  }
+  get rules() {
+    return this._rules;
+  }
+  extend(key, rules, prepend) {
+    if (!this._rules[key]) this._rules[key] = [];
+    if (prepend) {
+      this._rules[key] = rules.concat(this._rules[key]);
+    } else {
+      this._rules[key] = this._rules[key].concat(rules);
+    }
+  }
+  getMetadata(doc) {
+    return getMetadata(doc, this._rules);
   }
-};
+}
 
+MetadataParser.metadataRules = Object.assign({}, DEFAULT_RULESET);
+MetadataParser.buildRuleset = buildRuleset;
+MetadataParser.getMetadata = doc => getMetadata(doc, MetadataParser.metadataRules);
 module.exports = MetadataParser;